Telephone subscriber stations are generally equipped with ringers operable by alternating current of rather large amplitude and a low audio frequency, usually between 20 and 50 Hz, arriving from the exchange over the talking conductors of their respective lines. The riner and a blocking capacitor in series therewith are connected across these conductors in parallel with a hook switch which is normally open and whose closure, caused by a lifting of the handset, is detected by line-monitoring equipment which thereupon terminates the emission of ringing current and connects the station to the remote caller.
Modern telephone exchanges operate with electronic circuitry incapable of directly dealing with low-frequency ringing currents of the required intensity. Unless the subscriber stations associated with such an exchange have electronic ringers responsive to low-intensity call signals in a higher audio-frequency range (as disclosed in commonly owned application Ser. No. 229,174 filed Jan. 28, 1981 by Franco Pira et al), it is necessary to provide the exchange itself with circuitry for generating low-frequency oscillations, suitable for driving an electromechanical ringer, upon the reception of an incoming call.
Such an arrangement requires a periodic switchover of the conductors of a called subscriber from the line-monitoring equipment normally connected thereto, upon the occurrence of an incoming-call signal, to an oscillator operating at the proper ringing frequency. If, now, the subscriber lifts the receiver off the hook to close the line loop, this fact cannot be detected by the monitoring equipment while the line conductors are switched to the oscillator so that the line current reaching the speaker of the handset will ring into the subscriber's ear until the end of the signal period in progress. Moreover, since the same oscillator usually serves a number of subscriber lines associated with a given exchange, an undesirable overload may exist at such an instant if several stations are being called simultaneously.
This problem could be avoided by the use of ancillary circuitry designed to detect the closure of the line loop and to communicate this fact to the electronic signaling equipment, yet such a solution would require considerable modification of the normal line-monitoring circuits.